1. Field of the Invention
This invention relates to color cathode ray picture tubes, and is addressed specifically to improved factory means and processes for the manufacture of tubes having a tensed foil shadow mask. Color tubes of various types that have a tension foil mask can be manufactured by the process, including those used in home entertainment television receivers. The process according to the invention is particularly valuable in the manufacture of medium-resolution, high-resolution, and ultrahigh resolution tubes intended for color monitors.
The use of the foil-type flat tensed mask and flat faceplate provides many benefits in comparison to the conventional domed shadow mask and correlatively curved faceplate. Chief among these is a greater power-handling capability which makes possible as much as a three-fold increase in brightness. The conventional curved shadow mask, which is not under tension, tends to "dome" in picture areas of high brightness where the intensity of the electron beam bombardment is greatest. Color impurities result as the mask moves closer to the faceplate and as the beam-passing apertures move out of registration with their associated phosphor elements on the faceplate. The tensed mask when heated distorts in a manner quite different from the conventional mask. If the entire mask is heated uniformly, there is no doming and no distortion until tension is completely lost; just before that point, wrinkling may occur in the corners. If only portions of the mask are heated, those portions expand, and the unheated portions contract, resulting in displacements within the plane of the mask; i.e., the mask remains flat.
The tensed foil shadow mask is a part of the cathode ray tube front assembly, and is located in close adjacency to the faceplate. The front assembly comprises the faceplate with its screen which consists of deposits of light-emitting phosphors, a shadow mask, and support means for the mask. As used herein, the term "shadow mask" means an apertured metallic foil which may, by way of example, be about 0.001 inch or less in thickness. The mask must be supported under high tension a predetermined distance from the inner surface of the cathode ray tube faceplate; this distance is known as the "Q-distance." As is well known in the art, the shadow mask acts as a color-selection electrode, or parallax barrier, which ensures that each of the three electron beams lands only on its assigned phosphor deposits.
The conventional process of depositing patterns of color phosphor elements on the screening surface of a color picture tube faceplate utilizes the well-known photoscreening process. A shadow mask, which in effect functions as a perforated optical stencil, is used in conjunction with a light source to expose in successive steps, three discrete lightsensitive photoresist patterns on the screening surface. The shadow mask is typically "mated" to each faceplate; that is, the same mask is used in the production of a specific tube throughout the production process, and is permanently installed in the tube in final assembly. At least four engagements and four disengagements of the mask, as well as six exposures, are required in the standard screening process. In certain processes, a "master" may be used for exposing the photoresist patterns in lieu of the mated shadow mask.
2. Prior Art
There have been a number of disclosures of tensed foil masks and means for applying tension to the mask and retaining the mask under tension. Typical of these is the disclosure of Law in U.S. Pat. No. 2,625,734 which addresses the construction of a taught, planar foraminous mask. A foil mask blank is loosely mounted in a two-section frame, and the mask is expanded by the hot-blocking process. Machine screws peripheral to the frame provide for clamping the mask tightly in the frame when the mask is in its expanded state. The mask becomes tensed upon cooling as it is restrained from returning to its former dimensions by its captivation by the frame. The frame with the mask enclosed is mounted with the phosphor-bearing screen as a unitary assembly adjacent to the inner surface of the faceplate. Law in U.S. Pat. No. 2,654,940 also discloses means for stretching and captivating by frames masks formed from wire mesh.
U.S. Pat. No. 3,894,321 to Moore, of common ownership herewith, is directed to a method for processing a color cathode ray tube faceplate in conjunction with a thin foil tension shadow mask. A frame screw-clamp supports a tensed mask during lighthouse exposure of an associated screen. The faceplate is registered with the mask support frame by means of three alignment posts which extend from the lighthouse, and against which the frame and the faceplate are both biased by gravity. The faceplate and frame, being both referenced to the three lighthouse posts, are thereby referenced to each other.
In U.S. Pat. No. 4,591,344 to Palac, of common ownership herewith, a method of making a color cathode ray tube is disclosed in which a frame on which a shadow mask is stretched has indexing means cooperable with registration-affording means on a faceplate. The assembly provides for multiple registered matings of the faceplate and mask during photoscreening operations. A photographic plate is used in a process for applying the phosphor elements to the faceplate screening surface to provide an interchangeable mask system; this in lieu of the more common method of using a shadow mask permanently mated with a faceplate, and which serves as an optical stencil during the photoscreening process. The sealing areas of the faceplate and the frame are joined in a final assembly operation such that the frame becomes an integral constituent of the cathode ray tube.
A mask registration and supporting system for a cathode ray tube having a rounded faceplate with a skirt for attachment to a funnel is disclosed by Strauss in U.S. Pat. No. 4,547,696 of common ownership herewith. The skirt of the faceplate provides the necessary Q-distance between the mask and the screen. A frame dimensioned to enclose the screen comprises first and second space-apart surfaces. A tensed foil shadow mask has a peripheral portion bonded to a second surface of the frame. The frame is registered with the faceplate by ball-and-groove indexing means. The shadow mask is sandwiched between the frame and a stabilizing or stiffening member. Following final assembly, the frame is permanently fixed in place within the tube envelope between the sealing lands of the faceplate and a funnel, with a stiffening member projecting from the frame into the funnel.
In U.S. Pat. No. 4,593,224 to Palac, of common ownership herewith, there is disclosed a shadow mask mount in the shape of a rectangular frame for use in tensing an in-process shadow mask, and for temporarily supporting the mask while in tension. An apertured foil comprising the in-process mask is laid across the opening in the frame and is secured to the frame by brazing or welding. The coefficient of thermal expansion of the foil is preferably equal to or slightly less than that of the frame. A glass frame is also provided that consists of two identical rectangular members smaller in circumferential dimension than the metal frame. When joined into a single frame, the members are located between the tube faceplate and funnel to become an integral part of the tube envelope in final assembly. Each member of the glass frame has indexing means, one member for indent-detent registration with the faceplate, and the other for indent-detent registration with the funnel. Following the application of a layer of devitrifying cement in paste form to the facing surfaces of the two members, the mask, held in the metal frame, is sandwiched between the two members. As the assembly is heated, the expansion of the mask is taken up by screw means attached to the metal frame which press against the peripheries of the members. Upon cooling of the assembly, the coefficient of thermal expansion of the mask, being greater than that of the glass, results in the mask being held permanently in tension by the glass frame through the medium of the frit cement, which has become solidified by the heat. The portion of the mask that projects beyond the periphery of the glass frame is severed to release the metal frame. The glass frame with its captivated mask is then mounted on a lighthouse for photoscreening the faceplate, with registration with the lighthouse and faceplate provided by the indent-detent means described.
In referent U.S. Pat. No. 4,721,488 of common ownership herewith, there is disclosed an apparatus for tensing a foil shadow mask. The apparatus comprises a pedestal having registration-affording means, and a tensing structure which includes a fixture comprising a pair of collars for clamping the edge of a foil to support and maintain the foil in a taut condition. An anvil is provided for engaging a peripheral portion of the clamped foil to induce deflection of the foil, and thereby, a predetermined tension in the foil. Following a photoscreening process, the mask is secured to shadow mask supports extending from the faceplate by welding.
In a journal article, there is described means for mounting a flat tensed mask on a frame for use in a color cathode ray tube having a circular faceplate with a curved viewing surface. In one embodiment, the mask, which is also circular, is described as being welded to a circular frame comprised of a 1/8 inch steel section. The frame with captivated mask is mounted in spaced relationship to a phosphor-dot plate, and the combination is assembled into the tube as a package located adjacent to the faceplate. ("Improvements in the RCA ThreeBeam Shadow Mask Color Kinescope," by Grimes et al. The IRE, January 1954; decimal classification R583.6.)